A very common question when discussing modern SharePoint is why Microsoft has not modernized the creation of subsites to give Office 365 tenants the ability not only to create a modern site, but also to create modern subsites. Fortunately, Microsoft has finally released a modern subsite template to allow any organization still using subsites to create modern subsites and not get stuck with classic subsites created under a modern site.
You can find a quick sample about modern subsite creation in this link. For more information about SharePoint Online and OneDrive for Business, see Chapter 8 in Office 365 for IT Pros